1. Field of the Invention
The present invention relates to digital cameras employing solid-state pixel sensors. More particularly, the present invention relates to solid-state pixel sensor arrays having improved sensitivity and auto-exposure detection apparatus for use in digital cameras.
2. The Prior Art
Storage pixel sensors are known in the art. These devices sense photo-integrated charge stored on a capacitor. A limitation of these prior pixels storage sensors is the tradeoff that results from the desire for the capacitor to be small to achieve high sensitivity and the competing desire for the capacitor to be large to provide low noise and good storage time.
Prior art exposure control techniques known to the inventors that use the actual image sensors during the actual exposure interval are of two types. Some prior art techniques integrate the total photocurrent by a common back-side electrode (anode) of a group of photodiodesxe2x80x94i.e., they integrate the substrate current to get an average light reading on the whole array. Other prior art techniques use nondestructive readout to sample selected pixels during the exposure interval, looking for an indication that some pixels are reaching a full-scale exposure. Another prior-art technique senses a total overflow charge from the pixel sensors.
The first technique is tricky and difficult to implement, since the photocurrents are small and the substrate is large and noisy. In addition, it responds strictly to the average light level across the image plane rather than to those pixels that are reaching a full-scale charge accumulation. The second technique requires a sequential polling, so is limited to either a very slow operation or to sensing only a very small subset of the pixels. The second technique is therefore not good for detecting the exact time when a small percentage of pixels are reaching a full-scale exposure. The third technique requires sensing of charges against a background of the total leakage of the full area of pixel sensors.
Other prior art techniques for exposure control typically measure the light either at a different time, e.g. just before the actual exposure, or with a different sensor device that needs to be calibrated relative to the sensor that is picking up the actual image. Such techniques typically sample the image plane at selected fixed points rather than adapting to the lighting conditions of the entire image.
One such prior art technique uses an imager first to estimate a light level and thereby to calculate an optimum exposure duration for a second cycle of the imager. This technique is obviously not as fast, and particularly is unsuited to controlling the exposure time rapidly during a dynamic lighting event, provided for example from a strobe flash.
Another such prior art technique employs a separate overall light sensor to measure an average light level and to react to a sufficient quantity of light by closing a shutter or quenching a strobe flash. Mechanical shutters and non-frame-storage electronic sensors cannot be shuttered rapidly enough to use this technique during a flash, which is why the detector is sometimes used to turn off the light source instead of closing a shutter. These techniques require an awkward coordination between the camera, the light sensor, and the light source, and do not necessarily track automatically the sensitivity (or film speed) and lens aperture of the camera.
Another type of prior art technique relates to use of an adjustable overflow drain for dynamic range enhancement. These techniques have not been integrated with the use of the overflow current for terminating the exposure time. Variations on this technique employ either a moving overflow barrier or a dual exposure interval to increase dynamic range.
According to a first aspect of the present invention, an array of MOS active storage pixel sensors disposed on a semiconductor substrate is disclosed. Each pixel sensor in the array comprises a photodiode having a first terminal coupled to a first potential (ground) and a second terminal. A reset transistor hasing a first terminal coupled to the second terminal of the photodiode, a second terminal coupled to a reset reference potential that reverse biases the photodiode, and a control gate coupled to the reset line. A photocharge integration node comprises the gate of a first source-follower transistor having a drain, coupled to a first source-follower drain line, and a source. According to the present invention, the pixel sensor may comprise combinations of other elements in addition to the photodiode. Different embodiments of the invention employ one or more of a barrier transistor, a source-follower bias transistor, a transfer transistor, a saturation level transistor and an output amplifier transistor.
One embodiment of the present invention includes a barrier transistor having a first terminal coupled to the second terminal of the photodiode and a second terminal coupled to a photocharge integration node comprising the gate of the first source-follower transistor.
In another embodiment of the present invention, the first source-follower transistor is coupled to a bias current source and has an output. A bias transistor has its drain coupled to the output of the first source-follower transistor, its gate coupled to a fixed potential to establish the saturation level along with the fixed voltage on the gate of the saturation level transistor and a source coupled to a fixed voltage such as ground.
In another embodiment of the present invention, a semiconductor transfer transistor has a first terminal coupled to the output of the first source-follower transistor and a second terminal connected to a capacitive storage node comprising the control element of a second source-follower transistor having an output. A row-select transistor has a first terminal coupled to the output of the second source-follower transistor, a second main terminal coupled to a column output line and a control element coupled to a row-select line.
The separation of the photodiode from the charge integration node by the barrier transistor allows high sensitivity, and the separation of the storage node from the charge integration node by the first source-follower transistor allows low noise storage and readout by providing a smaller capacitance for the charge integration node and a larger capacitance for the storage node.
According to another aspect of the present invention in which the pixel sensors each employ a bias transistor, an auto-exposure circuit for use with pixel sensors is disclosed. A saturation level transistor has its source coupled to the output of the first source-follower amplifier, its gate coupled to a fixed potential chosen to turn the transistor on at a preselected pixel voltage representing a saturation level, and a common (global) current summing drain node. A bias transistor has its drain coupled to the output of the first source-follower amplifier, its gate coupled to a fixed potential to establish the saturation current and a source coupled to a fixed voltage such as ground. The global current summing node is coupled to a current comparator to compare the current flowing from the common drain node with a reference current. When the current from the global drain node exceeds the reference current, indicating that a preselected number of pixels in the array have saturated, the output of the comparator produces a TERMINATE EXPOSURE signal.